This invention relates, in general, to resonators and oscillators, and more particularly to piezoelectric resonators and oscillators which can be integrated with other electronic devices.
In the design of radio receivers, particularly paging receivers, cellular radios, and microwave satellite communication systems, it is desirable for components which form the system to take up as little space as possible. It is desirable for as many components as possible to be integrated into a single integrated circuit. This integration also reduces connections needed to make the radio, greatly improving reliability and reducing manufacturing cost.
Besides reduced size, higher and higher operating frequencies have become more common. This has led to the use of semiconductor materials which can operate in the gigahertz (GHz) frequency range to be used for the electronic components. Integrated circuits manufactured in gallium arsenide can operate at these frequencies. One problem that is basic to the operation of a high frequency radio, however, is generation of a high frequency oscillating electric signal which is used to both transmit and receive information. Similarly, resonator circuits which can be used as high frequency filters in the gigahertz frequency range are needed.
It has been known for some time that certain crystalline materials have piezoelectric properties. Specifically, there is what is called a direct piezoelectric effect, in which electrical charges appear on crystal surfaces upon the application of an external stress. There is also a converse piezoelectric effect, in which the crystal shows strain or deformation when an electrical charge is applied by external means to faces of the crystal. These effects have been used for many years in crystal oscillators and other devices in which bulk acoustic waves are transmitted through a crystal, typically between electrode plates at opposite faces of the crystal.
Usually, quartz crystals are used to make high frequency oscillators and resonators. These quartz oscillators are called bulk acoustic wave devices because acoustic waves are propagated throughout the bulk of the crystal. However, quartz crystal oscillators cannot be integrated with other components, and so must be coupled to other components on PC boards or hybrid substrates. Also, the technology of quartz crystal oscillators limits their ability at higher frequencies. Use of bulk waves in this manner has provided crystal oscillators and filters with good temperature stability, but with frequencies limited to about 200 megahertz (MHz) due to excessive capacitance, and more typically falling below 50 MHz. Consequently, higher frequencies cannot be obtained without the expense of more components, such as frequency multipliers. Also, when high frequencies are derived from multipliers, accuracy and stability are sacrificed. Because cost, accuracy, and size of frequency multiplier circuitry is proportional to multiplication necessary, it is advantageous to use as little frequency multiplication as possible.
Besides bulk acoustic wave devices, other piezoelectric devices, also called electroacoustic devices, fall into two basic categories: surface acoustic wave (SAW) and shallow bulk acoustic wave (SBAW). As their names imply, each of the variations refers to the location of the acoustic wave with respect to a piezoelectric film: at the surface or just below the surface. Devices using SBAW have become increasingly popular because they are more stable than SAW devices and can operate at higher frequencies.
Acoustic waves are generated in a piezoelectric film by providing an electric field across the film. Since most piezoelectric films are insulators, it is easy to establish an electric field across the film. Vertical waves can be established by an electric field across opposed surfaces of a piezoelectric film, while lateral waves can be established by two electrodes on a single surface. If an oscillating or pulsing electric field is supplied across the film, an oscillating acoustic wave will be established. To make an acoustic wave oscillator/resonator, a standing acoustic wave must be established in the film. Frequency of the standing acoustic wave will be a function of device geometry and physical properties of the piezoelectric material.
Techniques are also known to micromachine silicon structures to form diaphragms, beams, and cantilever beams which can then oscillate when an acoustic wave is established in them. Since silicon is not a piezoelectric material electroacoustic devices could only be made by forming a piezoelectric layer on top of a micromachined silicon structure. These micromachine structures allow higher frequency operation because of the smaller geometries used, as the structures can be formed using conventional semiconductor processing techniques. However, since a piezoelectric film was deposited on top of a non-piezoelectric semiconductor structure, even when micromachining was used the piezoelectric film was supported by an otherwise quiescent structure. Mechanical coupling between the piezoelectriC film and a non-piezoelectric material results in damping of the acoustic wave and lowered quality factor (Q) of acoustic wave filters and oscillators. Until now, devices which use an unsupported piezoelectric layer have not been available.
Accordingly, it is an object of the present invention to provide a method for generating an oscillating current of very high frequency.
It is a further object of the present invention to provide a piezoelectric resonator which can be monolithically integrated with other semiconductor devices.
A further object of the present invention is to provide a resonator with improved reliability.
Another object of the present invention is to provide a piezoelectric resonator wherein the piezoelectric element is not supported by a non-piezoelectric material.
Another object of the present invention is to provide an electroacoustic resonator/oscillator using a piezoelectric semiconductor material.
A further object of the present invention is to provide an electroacoustic resonator which uses a Schottky gate to establish an acoustic wave.